This example uses two modules instantiated many times. It be executed with MPI
(if mpi4py is available) or with the Python multiprocessing library. These choices
are made by variables listed below in the executable portion of this run file.

To run this case using MPI you should compute the number of
processes as follows:

nprocs = n_droplets + 1 vortex + 1 cortix

then issue the MPI run command as follows (replace nprocs with a number):

mpiexec -n nprocs run_droplet.py

To run this case with the Python multiprocessing library, just run this file at the
command line as